3 Description

F08FTF (ZUNGTR) is intended to be used after a call to F08FSF (ZHETRD), which reduces a complex Hermitian matrix A to real symmetric tridiagonal form T by a unitary similarity transformation: A=QTQH. F08FSF (ZHETRD) represents the unitary matrix Q as a product of n-1 elementary reflectors.

On entry: further details of the elementary reflectors, as returned by
F08FSF (ZHETRD).

6: WORK(max1,LWORK) – COMPLEX (KIND=nag_wp) arrayWorkspace

On exit: if INFO=0, the real part of WORK1 contains the minimum value of LWORK required for optimal performance.

7: LWORK – INTEGERInput

On entry:
the dimension of the array WORK as declared in the (sub)program from which F08FTF (ZUNGTR) is called.

If LWORK=-1, a workspace query is assumed; the routine only calculates the optimal size of the WORK array, returns this value as the first entry of the WORK array, and no error message related to LWORK is issued.

Suggested value:
for optimal performance, LWORK≥N-1×nb, where nb is the optimal block size.

Here A is Hermitian and must first be reduced to tridiagonal form by F08FSF (ZHETRD). The program then calls F08FTF (ZUNGTR) to form Q, and passes this matrix to F08JSF (ZSTEQR) which computes the eigenvalues and eigenvectors of A.